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Homework Help: Pretty easy relative max problem calc 3

  1. Dec 5, 2012 #1
    1. The problem statement, all variables and given/known data

    Locate all relative max,min,and saddle points if any.




    Skipping some algebra I get the critical points (2,-1)




    I know I'm messing up at f(x0,y0)

    I'm simply plugging in my c.p points (2,-1) into


    but seem to keep getting -3

    While the value 1 is suppose to come out

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Dec 5, 2012 #2


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    It looks like you have your formula for d wrong. It's supposed to be d=fxx*fyy-(fxy)^2. fxy is the mixed second derivative.
  4. Dec 5, 2012 #3
    I'm a bit confused what do you mean mixed second derivative?
  5. Dec 5, 2012 #4


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    First take the derivative of f with respect to x to get fx. Then take the derivative of fx with respect of y to get fxy. That's the mixed second derivative. Or you can do it in the other order and get fyx. They should be the same.
    Last edited: Dec 5, 2012
  6. Dec 5, 2012 #5
    Got it, thanks Dick.
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